1. Field of the Invention
The present invention relates to an inverter device for driving a brushless motor used in an electric power steering device in a vehicle, for example, and more particularly, relates to technique for detecting a phase current flowing in a motor.
2. Description of the Related Art
An electric power steering device in a vehicle is provided with a motor such as a 3-phase brushless motor, in order to apply to a steering mechanism a steering aiding force corresponding to a steering torque of a handle. The steering torque applied to the handle is detected by a torque sensor and, according to a detected value thereof, a target value of an electric current to be flowed through the motor is calculated. Then, based on a difference between the target value and the value of the electric current actually flowing through the motor, a command value for feedback control to be supplied to a motor driving portion is calculated. A motor driving circuit includes a PWM circuit for creating a PWM (Pulse Width Modulation) signal having a duty ratio corresponding to the command value, and an inverter circuit including pairs of upper and lower switching devices provided for respective phases, wherein the pairs of upper and lower switching devices are on or off according to the duty ratio of the PWM signal output from the PWM circuit. The inverter circuit outputs voltages for the respective phases corresponding to the aforementioned duty ratio, based on the on/off operations of the switching devices, to drive the motor with the voltages. The electric currents in the respective phases of the motor are detected by determining the voltages at opposite ends of a phase-current detection resistance connected in serial to the switching device, and the detected value is the value of the electric current actually flowing through the motor. Detection of a phase current in such a PWM type motor driving device is described in, for example, Japanese Patent Publication No. 3245727, Japanese Patent Publication No. 3240535, Japanese Patent Application Laid-Open No. 2005-1574, and Japanese Patent Application Laid-Open No. 2003-164159.
Japanese Patent Publication No. 3245727 describes a motor driving device including a 3-phase brushless motor, an inverter circuit for driving the motor, a PWM circuit for performing PWM control on the inverter circuit, and resistances for detecting phase currents flowing through the motor, wherein there is provided a sample-and-hold circuit for sampling and holding voltages across phase-current detection resistances for respective phases, using PWM signals from the PWM circuit as sampling signals, and outputs the sampled-and-held signals as phase-current detection signals.
Japanese Patent Publication No. 3240535 describes provision of a circuit for generating a timing signal in addition to the structure of Japanese Patent Publication No. 3245727, so as to, based on the timing signal from the circuit, start sampling of the voltages across the phase-current detection resistance later than a timing when switching devices in the inverter circuit are on and stop sampling of the voltages across the phase-current detection resistances earlier than a timing when the switching devices in the inverter circuit are off.
Japanese Patent Application Laid-Open No. 2005-1574 and Japanese Patent Application Laid-Open No. 2003-164159 describe that, when the voltage across the phase-current detection resistance can not be detected in any of the phases of the inverter circuit, since the duty ratio of the PWM signal applied to the lower switching device is less than a predetermined value, the phase current in the phase is calculated from the phase currents in the other two phases, by utilizing the fact that the total sum of the respective electric currents in the three phases is zero.
On the other hand, the inverter circuit controlled by the PWM signal includes pairs of upper and lower arms corresponding to the respective phases, each of the arms being provided with a switching device. If a process of switching the switching devices in the upper and lower arms between on/off states involves an instant when both the switching devices are on at the same time, a short circuit may be caused between the upper and lower arms, which may allow an excessive electric current to flow through the respective switching devices, thereby resulting in destruction of the switching devices. Therefore, such that one of the switching devices in the upper and lower arms is off and thereafter the other switching device is on, a time difference called a dead time is provided between the on/off timings of both the switching devices for protecting the switching devices. Such a dead time is described in, for example, Japanese Patent Application Laid-Open No. 2003-324928 and Japanese Patent Application Laid-Open No. 11-183531.